1. Field of the Invention
The present invention relates to a magnetically driven agitator and to a process for adjusting the couple limiting transmission of effort of such an agitator.
2. Brief Description of the Related Art
Agitators are conventionally used for stirring a mixture contained inside a recipient in order to avoid decantation or any other alteration of the mixture in the course of time. A magnetically driven agitator has the advantage that the propelling screw which it comprises is set in motion by a magnetic coupling which occurs without physical contact between two rotating parts of which one is driven by the driven shaft of an electric motor while the other is constituted by a propelling screw. This makes it possible to arrange the part associated with the shaft of the electric motor outside the recipient while the propelling screw is installed inside the recipient. Any danger of leakage at the level of the agitator may thus be set aside. This is particularly useful when the mixture is toxic or when pollution thereof by outside agents is to be avoided, such as for example in the case of a medicinal composition.
The magnetic coupling used for an industrial agitator must be intense in order to drive the propelling screw of the agitator with a sufficient force. Now, it is sometimes necessary to proceed with dismantling of the propelling screw, in particular for reasons of maintenance or inspection of the recipient in which the mixture is formed. It is thus standard to provide cleaning the propelling screw of an agitator and/or sterilizing it outside the recipient at the end of each production batch. It is sometimes necessary to dismantle the propelling screw in order to proceed with the standard exchange of wear pieces such as bearings. When a propelling screw has been dismantled, it must be returned into position on its support with the greatest precautions, avoiding as much as possible knocks that might damage the blades, the bearings and/or the surface of the recipient.
The magnetic forces necessary for driving the propelling screw of an industrial agitator are such that the effort that an operator must exert to remove the propelling screw is considerable, as this effort must overcome both the weight of the propelling screw and the force of magnetic coupling necessary for the drive. This is even more critical when the propelling screw is replaced in position, insofar as, when it is being installed offered, it may happen that the magnetic effort is so intense that the propelling screw escapes the operator's grip and is violently applied against its support, consequently damaging the bearings and even injuring the operator.
In addition, it is particularly delicate for an operator to place the propelling screw in perfect alignment with the axis of rotation of its rotor, with the result that, if the propelling screw escapes the operator's grip due to the magnetic force that it undergoes, it tends to be applied aslant on its support, which may lead to the destruction of one of its blades, to the marking of the inner surface of the tank and/or to one of the bearings being damaged. In order to overcome this drawback, it may be envisaged systematically to dismantle the drive assembly of the propelling screw located outside the recipient, i.e. it drive motor and possibly the reduction gear which is associated therewith, in order to eliminate the magnetic forces exerted on the propelling screw in the course of assembly or dismantling. Such an approach requires that an operator manipulate heavy and cumbersome parts, these parts generally being located under the manufacturing tanks or recipients and being difficult to access. In addition, such a dismantling of these drive systems must be followed by re-assembly during which the axes of the rotating parts must be very precisely aligned, which is not always possible taking into account the difficult access to the zones of re-assembly of the motor and its possible reduction gear. Moreover, dismantling of the outer part of the agitator involves exposing the magnetic drive rotor to the open air, this rotor being provided with permanent magnets of which the outer surfaces may be covered with magnetic impurities. Taking into account the small clearance present around the rotor, these impurities may lead to a machining of the magnets and to a blockage of the agitator.
It is a particular object of the present invention to overcome these drawbacks by proposing a magnetically driven agitator of which the propelling screw may be easily dismantled and returned into place, without the risk of the magnetic forces disturbing these operations and without requiring the complete dismantling of the part of the agitator outside the recipient on which it is mounted.